I know that Airline Revenue Systems are more complicated to understand than the meaning of life, but can anyone explain the following:

LHR-LAX
2nd March 09, returning 12 March 09.

BMI Price/BMI Code Share on UA Metal: Ł280.70. WLXBD2.
UA Price/UA Flight #'s/Same UA Metal: Ł502.70. WLXELF25.

Obviously different fare basis even though both are W fares - is this just BMI trying to sell early fares? Surely they revenue share with UA on this?

Furthermore, an infant fare on BMI is only an extra Ł32.70, whereas according to the UA website you have to buy a child seat if flying International as an infant?

Obviously my money is going to be going to BMI, as there seems no benefit whatsoever paying UA direct for this, just looking for some clarification if there is some for these results?

The website is confusing, but no. The child has to have "a ticket," but that doesn't mean they have to have a seat. It just means that you have to pay for the lap infant for int'l travel, whereas a lap infant travels for free on domestic flights. I believe the cost is 10% of the ticketed cabin's full fare, but I'm not positive about this, having never done it.

As for the discrepancies between prices for the same flight... that's just how codeshares are sometimes, and I have no idea why. I believe the non-operating carrier basically purchases some number of seats in bulk from the operating carrier, and they can then sell them for whatever they like. Of course, I'm just guessing here.

Each selling carrier sets its only pricing and yield management. @:-)

You can only upgrade in advance if you book direct with UA flight number. @:-)

10% is correct, for children under 2. Over 2 you have to buy a seat, but still get a slight discount.

I've found the sme scenario for travel in Sept. UA is $300 more then the BMI codeshare ironically, the fareclass was a W on the BMI flight and an S on the UA. It's a gamble, but I'll buy the BMI one and hope to clear the upgrade at the gate. But conversely, there are many dates where BMI is $300 higher then the UA metal. Go figure, what a kooky business.

How about if one is traveling in business on an award seat -- is it 10% of the business class fare or economy fare?

It is 10% of the cabin travelled. Yes, it could be over $1000 for first class. You are better off redeeming an additional seat for the little one sometimes.

Or, on behalf of your fellow travelers, sitting in a lower class of service with your crying baby.

I think it's 10% of ticketed class (I haven't done it, but did think about it). If you buy Y and upgrade to C, it's 10% of Y. If you buy C with cash or miles, it's 10% of C. In either case, I believe it's 10% of full fare, not the fare you paid.